This invention relates to a shadow mask for a color cathode-ray tube, more particularly to a curved shadow mask with thickness variations adapted to prevent localized doming.
Electron beams impinging on a shadow mask cause it to heat and expand. When the heating is localized it can cause part of the shadow mask to bulge outward, a phenomenon known as localized doming, the visible result of which is color misregistration in a local area on the screen. This problem is particularly noticeable in cathode-ray tubes with wide deflection angles. It is a major concern in the design of recent television sets with 110.degree. deflection angles and relatively flat screens. A variety of measures have been taken to prevent or reduce localized doming, but none have been satisfactory.
Instead of being made of steel, for example, a shadow mask can be made of Invar, a nickel-steel alloy with a low coefficient of thermal expansion. Unfortunately, use of Invar increases the material cost and the cost of the manufacturing process while reducing the manufacturing yield. The anti-doming effect is moreover not as great as hoped for, because while Invar has only 1/9 the coefficient of thermal expansion of steel, it also has only 1/5 the thermal conductivity. Thus, heat cannot easily escape from localized areas. In practice, the use of Invar reduces doming by only about seventy percent, which is inadequate.
Another possible countermeasure is to coat the back surface of the shadow mask with a heavy metal oxide such as lead oxide or bismuth oxide. The heavy metal atoms tend to reflect electrons, thereby reducing heating of the shadow mask. Coating a steel shadow mask with such an oxide, however, reduces doming by only about thirty percent, which is far from adequate. Further, the manufacturing process becomes more difficult and costly because the oxide has an unwelcome tendency to clog apertures in the shadow mask.
Corrugation, which has been used to give rigidity to flat shadow masks, is also useful in preventing doming. A corrugated shadow mask is bent in a repeating wave-like or step-like pattern that interferes with the formation of localized domes. Since a corrugated shadow mask necessarily departs from the ideal mask geometry, however, there is an unavoidable loss of picture quality.
Formation of grooves that reduce the rigidity of the shadow mask in the direction of greatest curvature has also been proposed. However, this measure has turned out to be effective only under certain restricted conditions, and does not appear to be practical in general.